Diminishing world supplies and decreasing availability of crude oil as well as sporadic regional shortfalls of gasoline for motor fuel have created considerable incentive for the development and use of alternative fuels. Ethanol is gaining wide popularity as such a fuel, particularly when mixed with gasoline to form a mixture known as gasohol. Automobiles can run on gasohol containing up to about 10 volume percent ethanol without requiring engine modifications. Ethanol is also widely used as a chemical solvent and as a raw material in the manufacture of drugs, plastics, lacquers, polishes, perfumes and the like.
Ethanol is derived primarily from the fermentation of mash, usually from corn and/or sugar cane. Natural fermentation is capable of producing an alcohol/water product mixture containing up to about 12 mole percent ethanol. Subsequently, the ethanol is separated from the ethanol/water product mixture via distillation which requires a substantial amount of energy because the mixture forms an azeotrope at a liquid phase composition of about 90.0 mole percent alcohol. Considerable investigation is being conducted to devise a more energy efficient and less capitally intensive process for breaking the ethanol/water azeotrope.
Several processes are known in the art for breaking the ethanol/water azeotrope to form motor fuel grade alcohol containing about 98.0 mole percent alcohol. For example, U.S. Pat. No. 4,319,058 discloses a process for separating ethanol from an ethanol/water feed mixture which comprises contacting the mixture with an adsorbent comprising a shaped replication of particle aggregates comprising carbonaceous pyropolymers containing recurring units of at least carbon and hydrogen atoms at a temperature ranging from about 20.degree. to about 230.degree. C., and a pressure ranging from about atmospheric to about 500 psig, selectively adsorbing substantially all of the ethanol to the substantial exclusion of water and thereafter recovering high purity ethanol by passing a desorbing material over the adsorbent.
U.S. Pat. No. 4,343,623 discloses an adsorptive separation process for separating ethanol from a feedstock mixture comprising ethanol and water. The process comprises contacting the feedstock mixture with an adsorbent comprising esterified silica, selectively adsorbing substantially all of the ethanol to be separated to the substantial exclusion of the water and thereafter recovering high purity ethanol. A desorption step may be used to desorb the adsorbed ethanol from the adsorbent.
U.S. Pat. No. 4,382,001 discloses a process for separating ethanol from a feedstock mixture comprising ethanol and water. The process comprises contacting the feed mixture with an adsorbent comprising activated carbon, selectively adsorbing substantially all of the ethanol to be separated to the substantial exclusion of the water and thereafter recovering high purity ethanol. A desorption step is used to desorb the adsorbed ethanol wherein the desorbent is capable of direct blending into motor fuel.
U.S. Pat. No. 4,407,662 discloses a process for removing sufficient water from an ethanol/water mixture of at least 160 proof to produce ethanol having a proof of 195 or more comprising the steps of heating the ethanol/water mixture until it is a vapor with sufficient super heat to maintain the vapor phase and to prevent substantial capillary adsorption as the mixture passes through a dessicant bed of molecular sieves, passing the super heated ethanol/water mixture through the bed to remove sufficient water to increase the proof of the ethanol to at least 195, passing a portion of the dehydrated ethanol through a second dessicant bed of molecular sieves at less than atmospheric pressure to desorb the water and ethanol on the dessicant from a previous dehydration cycle and reversing the flow through the two beds after the temperature of the first bed increases no more than about 14.degree. C.
An article by Mssrs. Garg and Yon, (Chemical Engineering Progress, Vol. 82 No. 2, pg. 54, February 1986) discloses an adsorptive heat recovery drying system for the energy efficient drying of petrochemical streams containing a substantial amount of water. The process comprises withdrawing a vapor stream containing about 83-90% alcohol from a top tray of a beer column and super heating the stream to prevent condensation. The super heated vapor stream is then dried by selectively adsorbing the water onto a solid adsorbent. The adsorbed water is then desorbed so that the adsorbent can be reused. The regeneration is effected by heating the adsorbent with a hot inert gas such as nitrogen or carbon dioxide in a closed loop recycling system carried out at elevated pressure and temperature.
A need exists in the art for an energy efficient, less capital intensive process for preparing motor fuel grade alcohol, and more particularly, to an energy efficient, cyclic process for recovering motor fuel grade alcohol from the ethanol/water product formed in conventional fermentation reactors.